1. Field of the Invention
The present invention relates to a vibration isolation apparatus for a stage which is designed to control vibration in a stage for exposure apparatus used in manufacturing semiconductor devices, liquid crystal display devices and the like.
2. Description of the Related Art
Conventionally, a lithography process in manufacturing semiconductor devices, liquid crystal display devices or the like uses an exposure apparatus (steppers and the like) which transfer a pattern on a mask (reticle or the like) onto shot areas on a substrate (wafer, glass plate or the like) coated with photoresist. For example, in an one-shot exposure type exposure apparatus like a stepper, a process of transferring a pattern on a mask onto each shot area on a substrate requires stable conditions attained by controlling or suppressing vibration of the mask and substrate. To meet this requirement, a surface plate is installed on a floor via vibration isolation bases or mounts held therebetween in order to prevent vibration being directly transmitted from the floor to a portion of the exposure apparatus located above the surface plate (i.e. an exposure unit).
Recently, to meet the need for transferring by exposure a pattern on a wider mask onto a substrate without increasing the size of a projection optical system, scanning exposure apparatus such as step-and-scan exposure apparatus are being used. These apparatus synchronously with scanning a mask in a direction perpendicular to the optical axis of a projection optical system, scans a substrate in a corresponding direction at the same velocity ratio as the magnification of the projection optical system, whereby the pattern on the mask is transferred by exposure onto the substrate in succession. In order to stably scan each of a mask and a substrate at a fixed velocity, it is also necessary that in the scanning exposure apparatus vibration is prevented from being transmitted from a floor to an exposure unit. This is accomplished by installing vibration isolation mounts therebetween.
A conventional vibration isolation apparatus for exposure apparatus will now be described. FIG. 13 shows a schematic construction exemplifying an exposure apparatus provided with a conventional vibration isolation apparatus. In FIG. 13, an exposure unit 11 including a wafer stage WS on which a substrate, i.e. a wafer 4 is mounted, a projection optical system 3, a reticle stage 2 on which a mask, i.e. a reticle 1 is mounted, an illumination optical system EL, columns 22, 23 for supporting those members, and a surface plate 9 for supporting the columns 22, 23 is supported on three or four vibration isolation bases or vibration isolation mounts disposed thereunder. FIG. 13 shows only two vibration isolation mounts 112a, 112b. Separate from the exposure unit 11, there is installed a control rack 28 housing a control system which controls the illumination optical system EL, the reticle stage 2, the wafer stage WS, and a handler (not shown) for loading/unloading the wafer 4 and the reticle 1.
The vibration isolation mounts 112a, 112b are 5 fixed on a base plate 13 so as to maintain their relative position. The vibration isolation mounts 112a, 112b are usually constructed from a combination of a spring material and a vibration damping material. A vibration isolation system as shown in FIG. 13 can be said to be a passive vibration isolation system which does not change vibration isolation performance in accordance with a state of vibration or a state (posture or the like) of the apparatus. Such a vibration isolation mount or vibration isolation base is usually referred to as "a passive vibration isolation mount".
FIG. 14 shows a schematic structure exemplifying another conventional exposure apparatus. In FIG. 14, the exposure unit 11 is also supported on a plurality of vibration isolation mounts as in the above-mentioned example. FIG. 14 shows only two vibration isolation bases or vibration isolation mounts 122a, 122b. The vibration isolation mounts 122a, 122b use an air spring (air damper). The exposure apparatus has an external air source which supplies a positive air pressure of 3 to 10 kgf/cm.sup.2 (gauge pressure) to air chambers sealed with rubber or the like of the vibration isolation mounts 122a, 122b through air pipings 126a and 126b, respectively, whereby air springs are constructed.
Flow control valves 124a, 124b are provided immediately before air inlets to the vibration isolation mounts 122a and 122b , respectively. The flow control valves 124a, 124b are adapted to operate in connection with level sensors 125a and 125b, respectively, which are mechanical or electrical space measuring instruments for detecting the posture of the exposure unit 11. That is, as the posture of the exposure unit 11 varies, air flow to the vibration isolation mounts 122a, 122b varies, whereby the exposure unit 11 is maintained in a fixed posture. Other features of FIG. 14 are the same as those of FIG. 13. The vibration isolation mounts 122a, 122b are also called "passive vibration isolation bases" as those in FIG. 13.
By contrast, "an active vibration isolation base" has recently begun to be used which detects a state of external or internal vibrations in real time using a sensor such as an accelerometer, displacement gauge or the like so as to positively affect performance of a vibration isolation mount.
The prior art, however, fails to provide vibration isolation mounts which are satisfactory in terms of both performance and cost.
That is, in an exposure apparatus, in addition to shaking and vibration transmitted from an external source, particularly from a floor, shaking and vibration generated by a stage operation of moving a member to be exposed such as a wafer or the like or a mask (reticle or the like) at a high velocity must be taken into consideration. When a stage is accelerated or decelerated, a large reaction force acts on the exposure apparatus because of the principle of action and reaction. The reaction force becomes a vibration source which transmits vibration to an exposure unit located on a vibration isolation mount. Physically, it is difficult to completely eliminate vibration without drastically modifying the structure of the exposure apparatus. A countermeasure for vibration of this kind is to increase the vibration damping capability as much as possible and to damp vibration as quickly as possible.
What has been discussed above may be summarized as follows. Functions which are required of a vibration isolation base or mount for exposure apparatus are: (a) reduction in the transmission of vibration from a floor and (b) rapid damping of vibration generated within an apparatus.
However, in view of the required performance of a vibration isolation base or mount, these two functions are incompatible with each other. That is, to attain function (a) "reduction of transmission of vibrations from a floor", a connection between an exposure apparatus and the ground or floor should be made as weak as possible; in other words, a "soft vibration isolation base" having a low rigidity is required. An example of this kind of base is an air spring type vibration isolation base or mount. On the other hand, to attain function (b) "rapid damping of vibration generated within an apparatus", a "hard vibration isolation base" having a high rigidity is required so that the exposure apparatus does not vibrate together with the ground or floor as an integrated unit. The latter function is provided by a vibration isolation mount which uses a mechanical spring having a high rigidity as a component thereof or by means of a rubber vibration isolator or the like.
Conventional passive vibration isolation bases or mounts described above, including vibration isolation mounts which use air springs as well as simpler and cheaper rubber vibration isolators or rubber cushions, have an advantage that a relatively satisfactory vibration isolating performance is obtained at relatively low cost. However, it is difficult for the conventional passive vibration isolation mounts to fulfill the above-mentioned vibration isolating functions which are required in exposure apparatuses.
As for active vibration isolation bases or mounts, sensors are installed inside and outside an apparatus in order to control vibrations based on detection signals issued therefrom to thereby fulfill the aforesaid two functions (a) and (b). Thus, by using the active vibration isolation bases, a vibration isolation system which fully satisfies various requirements for performance can be constructed. However, such a vibration isolation system requires various sensors having a sufficiently high precision and a controller having considerably complex electronic circuits for controlling vibration isolation mounts, leading to higher cost of apparatus. Particularly, a vibration isolation system of this kind which is applicable to recent high-performance exposure apparatus pushes up the cost of apparatus to unacceptably high levels.
On the other hand, as for aforesaid vibration dampers, i.e. vibration isolation mounts, air dampers or mechanical dampers formed of a compression coil spring contained in damping liquid are used. A vibration isolation mount itself has a centering function to a certain degree. Since an exposure apparatus requires an adjustment of height and level, each of four vibration isolation mounts is provided with a mechanical vertical motion mechanism. For example, when an exposure apparatus is relocated, the flatness and inclination of a floor changes, causing the inclination of an exposure unit to also change. To restore an original level, the height of the vibration isolation mounts is adjusted by means of the vertical motion mechanisms.
In the aforesaid conventional vibration isolation bases, by adjusting the vertical motion mechanism provided for each of four vibration isolation mounts, the height and level of the exposure unit can be adjusted. In this case, a plane is usually determined by three points, but the top ends of the four vibration isolation mounts are in contact with the bottom surface of a surface plate because the vibration isolation mounts can expand and contract vertically. However, as the vertical motion mechanisms are adjusted, the magnitude of expansion/contraction changes with each vibration isolation mount. This causes a change in balance among reaction forces which act from the vibration isolation mounts to the surface plate of an exposure apparatus. As a result, the surface plate deforms, leading to a problem of a deterioration in positioning accuracy or the like of a stage located on the surface plate.
Vibration isolation mounts, if provided, can shut off most vibrations transmitted from the floor to the exposure unit. However, it takes a relatively long time to damp the exposure unit, for example, by a stepping operation of a wafer stage. Accordingly, processing involves a time required for vibration to be damped, leading to a problem that the throughput (productivity) of an exposure process cannot be increased.